1. Field of the Invention
This invention relates to an optically active polysilane, an optically active film, and a method of controlling the optical activity of a solid thin film.
2. Description of the Related Art
It is strongly desired, with regard to twenty-first century information communication, to achieve the development of an ultra-miniature recording system which is capable of recording large quantities of digital information in a light-weight recording medium in a convenient and inexpensive manner and at high-speed. In the electric household appliances industries too, in view of the advent of commercial digital broadcasting, digital information recording devices such as digital video recorders, DVD-RAMs, and hard disk magnetic recorders are expected to be rapidly propagated from now onwards.
In the MO system, a minute change of around 0.15xc2x0 in optical Kerr rotation angle (the angle change of optical activity) occurs due to magneto-optical writing. Therefore, in order to achieve an excellent contrast/noise ratio, it is required to attach a reflecting mirror to the rear side of a recording medium to make it possible to utilize the change of optical Kerr rotation angle that has been amplified to about 0.3xc2x0 as a result of magneto-optical writing. For the purpose of detecting such a change in rotation angle which is as small as 0.3xc2x0, a detection mechanism, which is more accurate and larger in size compared with that of a magnetic head, is needed. The speed of reading/writing according to the MO system is around 30 milliseconds, which is three to five times slower than that of magnetic recording. One of the reasons for this is attributed to the fact that the detection head portion holding the beam splitter is relatively large in weight and size, thereby retarding the speed of the servo-track. Additionally, it is required to provide the recording device with a laser beam source and a magnetic head, thereby limiting miniaturization of the recording device as a whole.
If it is possible to develop a thin film material which is capable of optically writing and reading data as well as optically erasing the data, and also capable of exhibiting a very large change in optical rotatory power, which is much larger than 0.3xc2x0, as a result of the writing, it would become possible to take a step forward in realization of an all-optical recording system utilizing optical activity. As a result, it would become possible not only to further miniaturize the recording device but also to perform high-speed writing/recording which comparable with that of a hard disk.
At present, the magneto-optical recording (MO) of an inorganic magnetic thin film and the phase transition recording (PD) of an inorganic thin film are known as being useful as an optical recording material, and storage devices (recording materials) utilizing these principles are commercially available. The storage density in optical reading increases in inverse proportion to the square of the wavelength of laser employed. Therefore, when a short wavelength ultraviolet solid-state laser, for example, a GaN laser element (available from Nichia Kagaku Co., Ltd.) having an oscillation frequency of 370-430 nm in the ultraviolet region (presumed to be a light source of the next generation) is employed, it may be possible to realize a mass storage recording medium whose capacity is several times larger than that of the current DVD-RAM (laser beam source: 635 nm, 650 nm). Furthermore, if it is possible in the future to develop a material which is capable of complying with a laser beam source of much shorter wavelength (for example, a wavelength of 185-215 nm or a double wavelength of the GaN laser), it may be possible to perform high density recording ten times higher than that of current DVD-RAMs (laser beam source: 635 nm, 650 nm).
Meanwhile, an optical network for communication is constituted by a laser beam source (1300 nm, 1550 nm in central wavelength), a detector and an optical fiber forming a low loss window region, with the bandwidth of wavelength thereof being 1300 nm, 1550 nm.
If a material is found which is capable of developing and dissipating a fresh absorption in response to an external impulse in a wide wavelength region covering ultraviolet, visible and near infrared regions, the material is expected to be utilized as an optical recording material.
Therefore, an object of the present invention is to provide an optically active polysilane which is capable of exhibiting signals of large circular dichroism in a highly efficient manner over a wide wavelength region covering ultraviolet, visible and near infrared regions, and to provide an optically active film which can be formed using this polysilane.
Another object of the present invention is to provide a method of controlling the optical activity of a solid thin film relative to a light source selected suitably in conformity with a wide wavelength region covering ultraviolet, visible and near infrared regions.
According to one aspect of the present invention, there is provided an optically active polysilane represented by the following general formula: 
wherein R1 and R2 are a combination of groups selected from (R)-3,7-dimethyloctyl group and (S)-3-methylpentyl group, (S)-3,7-dimethyloctyl group and (R)-3-methylpentyl group, (S)-3,7-dimethyloctyl group and isopentyl group, (R)-3,7-dimethyloctyl group and isopentyl group, (S)-3,7-dimethyloctyl group and 2-ethylbutyl group, (R)-3,7-dimethyloctyl group and 2-ethylbutyl group, (S)-3,7-dimethyloctyl group and 2-cyclopentylethyl group, and (R)-3,7-dimethyloctyl group and 2-cyclopentylethyl group; R3 is an alkyl group having 3 to 20 carbon atoms and formed of a branched structure which is branched at any one of the first to fourth carbon atoms positioned away from the backbone chain; R4 is a straight-chain alkyl ether group having 2 to 22 carbon atoms, or a straight-chain alkyl group having 2 to 22 carbon atoms; x is a number ranging from 0.01 to 0.99; and wherein the number or repeating units in the optically active polysilane as represented by formula (1) ranges from 10 to 100,000.
According to another aspect of the present invention, there is provided an optically active film containing the polysilane represented by the following general formula (1): 
wherein R1 and R2 are a combination of groups selected from (R)-3,7-dimethyloctyl group and (S)-3-methylpentyl group, (S)-3,7-dimethyloctyl group and (R)-3-methylpentyl group, (S)-3,7-dimethyloctyl group and isopentyl group, (R)-3,7-dimethyloctyl group and isopentyl group, (S)-3,7-dimethyloctyl group and 2-ethylbutyl group, (R)-3,7-dimethyloctyl group and 2-ethylbutyl group, (S)-3,7-dimethyloctyl group and 2-cyclopentylethyl group, and (R)-3,7-dimethyloctyl group and 2-cyclopentylethyl group; R3 is an alkyl group having 3 to 20 carbon atoms and formed of a branched structure which is branched at any one of the first to fourth carbon atoms positioned away from the backbone chain; R4 is a straight-chain alkyl ether group having 2 to 22 carbon atoms, or a straight-chain alkyl group having 2 to 22 carbon atoms; x is a number ranging from 0.01 to 0.99; and and wherein the number or repeating units in the optically active polysilane as represented by formula (1) ranges from 10 to 100,000.
According to a further aspect of the present invention, there is provided a method of controlling the optical activity of a solid thin film, the method comprising:
forming a thin film containing an optically active polysilane represented by the following general formula (1) and exhibiting a helical inversion on a substrate at a temperature which is low enough to prevent the generation of the helical inversion;
heating the thin film to decrease the intensity of the circularly polarized light of the thin film; and
cooling the thin film to increase the intensity of the circularly polarized light of the thin film: 
wherein R1 and R2 are a combination of groups selected from (R)-3,7-dimethyloctyl group and (S)-3-methylpentyl group, (S)-3,7-dimethyloctyl group and (R)-3-methylpentyl group, (S)-3,7-dimethyloctyl group and isopentyl group, (R)-3,7-dimethyloctyl group and isopentyl group, (S)-3,7-dimethyloctyl group and 2-ethylbutyl group, (R)-3,7-dimethyloctyl group and 2-ethylbutyl group, (S)-3,7-dimethyloctyl group and 2-cyclopentylethyl group, and (R)-3,7-dimethyloctyl group and 2-cyclopentylethyl group; R3 is an alkyl group having 3 to 20 carbon atoms and formed of a branched structure which is branched at any one of the first to fourth carbon atoms positioned away from the backbone chain; R4 is a straight-chain alkyl ether group having 2 to 22 carbon atoms, or a straight-chain alkyl group having 2 to 22 carbon atoms; x is a number ranging from 0.01 to 0.99; and n is a number ranging from 10 to 100,000.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by the instrumentalities and combinations particularly pointed out hereinafter.